US patent application publication number 2007/0142584 filed Jan. 27, 2006, the contents of which are incorporated by reference herein, discloses certain cationic siloxane prepolymers that are able to form water extractable medical devices as well as methods of making the monomers. An example of a monomer made according to the prior synthetic approach is provided in Formula (I) below:
wherein n is an integer from 1 to about 300.
The method taught in US patent application publication number 2007/0142584 used to synthesize a methacrylate capped cationic siloxane (bromide counter ion) is shown below:

This reaction scheme requires the use of a large excess of the polymerization inhibitor 3,5-Di-tert-4butylhydroxytoluene (BHT) as well as a large excess of the reactant 2-(dimethylamino)ethyl methacrylate (DMAEMA). Another inhibitor which could be used is 4-methoxyphenol (MEHQ). Even though there is a large excess of DMAEMA, this reaction occurs at a very slow rate (100 hours at 60° C.) before product conversion nears 100%. In addition, the boiling point of DMAEMA is 182° C. Due to the cationic nature of the final product, the only way to remove the unreacted DMAEMA is with a combination of high vacuum and heat (stripping). Washing the material results in the emulsification and fractionation of the product. Also, since the product has methacrylate functionality, the stripping of the DMAEMA is problematic and often results in premature polymerization of the reaction product. This is especially the case as the reaction is scaled up. Therefore an improved method of synthesizing cationic siloxane prepolymers would be desirable
In addition, although monomers such as those claimed in US patent application publication number 2007/0142584 provide medical devices that are entirely suitable in some circumstances, it was determined that medical devices prepared from a monomer mix containing a higher amount of monofunctional siloxane methacrylate would be highly desirable. We have discovered that an iodo salt of a cationic siloxane prepolymer having the structural formula (II) shown below:
allows a greater amount of monofunctional siloxanyl methacrylate to be incorporated in the monomer mix than the bromo salt of a cationic siloxane prepolymer as shown in Formula (III)
wherein n equals 39.